1. Technical Field of the Invention
The present invention relates generally to the field of semiconductor devices and, more particularly, to electric fuse programming devices.
2. Description of Related Art
In integrated circuits, including CMOS integrated circuits, it is often desirable to be able to permanently store information, or to form permanent connections of the integrated circuit after it is manufactured. Fuses or devices forming fusible links are frequently used for this purpose. Fuses can be programmed to replace defective elements with redundant elements, for example. Further, fuses can be used to store die identification or other such information, or to adjust the speed of a circuit by adjusting the resistance of the current path.
For conventional programming of electrical fuses, a large current is passed through the fuse via a coupled transistor (programming transistor). Thus, the programming transistor must have the capacity to carry the current required to program the fuse. In order to achieve that capacity, the width of the programming transistor is appropriately chosen. For polysilicide fuses, the peak current required for programming can be of the order of 10 mA or more. This order of current requires a wide transistor acting as the programming transistor, hence, the area required for implementation can be significant even if the individual fuse element itself is small.
A conventional circuit for programming a fuse is shown in FIG. 1 in which the fuse 11 is connected between the drain of the programming transistor 13 and a supply voltage VDD. Initially, the resistance of the fuse 11 is small (typically in the range of 100 ohms). The fuse 11 is programmed by switching ON the programming transistor 13 with a pulse 15 applied to the gate. When the transistor 13 is turned ON, the transistor 13 starts to conduct and current flows through the fuse 11. The current flow causes the fuse 11 to be heated-up and, if sufficient current continues to flow through it, it becomes programmed by melting, electromigration or other mechanisms, resulting in a much higher resistance (i.e. programmed resistance).
In order to ensure that sufficient current flows through the fuse 11 for programming, the supply voltage must be sufficiently high and the transistor 13 in saturation (which is the conventional design) for carrying the programming current at the applied gate voltage. Since the programming current can be quite large (10 mA or more), the size of the programming transistor 13 becomes quite large (40 micron by 0.4 micron for gate oxide thickness of 6.5 nm), costing precious area on the silicon wafer.
Exacerbating the problem is the fact that most integrated chips include multiple fuses and programming transistors such as the conventional multiple fuse circuit shown in FIG. 2. In DRAM chips, for example, a few thousand fuses are generally used. For transistors conventionally operated in the saturation region, the large size combined with the large number of transistors can be quite costly because of the silicon wafer area consumed. Hence, it is desirable to find ways to minimize the silicon area being used for fuse implementation while also ensuring reliable fuse programming.
The present invention achieves technical advantages as an apparatus and method of programming an electrical fuse using a transistor (such as a MOSFET) which is operated in its breakdown region as opposed to its saturation region. With the programming transistor operated in the breakdown region, a much higher current is enabled than the associated saturation current for the same size transistor. Thus, a smaller transistor can be used for programming the fuse. Cooperative with transistor operation in the breakdown region, a dynamic current compliance device is used to limit the peak current to prevent damage that can result from excessive current flow through the transistor.